1. Field of the Invention
This invention relates to a ball joint comprising a socket, a ball received in the socket, a stud integral with the ball, a synthetic resin ball seat received in the socket to support the ball rotatably about its center and a cap for urging axially the ball seat against the ball to close an opening of the socket at the opposite side to the stud, and more particularly, to a ball joint used for a steering gear or a wheel suspension system of an automobile.
2. Description of the Prior Art
Ball joints are well known which eliminate play and have high rigidity so as to restrain shimmy, flutter or the like produced in a steering gear and wheel suspension system during travelling of an automobile and thereby improve the feel of steering experienced by the driver (e.g., Japanese Utility Model Laid-open No. Sho 57-19221). In such ball joints, a synthetic resin ball seat is given a fastening tolerance compressed by applying an axial compressive load. In this case, if the compressive elastic modulus of the ball seat is too high, the change in the fastening tolerance has a large effect on rotational friction torque so that a soft elastic synthetic resin is selected for the ball seat. Since the soft synthetic resin expands radially by axial compression, the rigidity is not necessarily improved unless its outer diameter is restrained, even if play is eliminated for the axial load. While the rigidity is improved by restraining the outer diameter, an effect similar to one of the ball seat itself having higher elastic modulus is obtained to present a possibility of providing an excessive rotational friction torque.
Even if the rotational friction torque is proper in room temperature, the ball seat expands larger than metal due to the difference of the expansion coefficient between metal and synthetic resin when atmospheric temperature rises, still bringing about a problem that the rotational friction torque will increase. Also, problems are encountered where the ball seat does not slide relative to a spherical surface of the stud, but rotates together with the stud about its axis to provide unstable rotational friction torque which shortens the life of the ball seat itself. To overcome these problems, stoppers are necessary to prevent the ball seat from rotating which have disadvantages in increasing the cost of the assembling operation or the like.